Crystal structure and feasibility of intramolecular proton transfer reaction of salicylaldazine

Abstract

The work describes characterization of salicylaldazine by IR, NMR and single crystal X-ray diffraction studies. The compound crystallizes in the monoclinic crystal system with the space group P2₁/n, a=8.554(3), b=6.338(2), c=11.864(5) Å, and β=107.89(2)°. There are two strong intramolecular hydrogen bonds of the type O-H...N and two weak hydrogen bridges of the type C-H...O which help the molecules to pack in a layered structure. Semi-empirical calculations have been performed to predict theoretically the feasibility of intramolecular ground and/or excited state proton transfer reaction/s. Potential energy curves have been generated in the ground (S₀) and the lowest excited (S₁) singlet states to judge the feasibility of intramolecular single and double proton transfer reactions. Semi-empirical (AM1) calculations suggest that only the excited state intramolecular single proton transfer process is favored both thermodynamically and kinetically. No other prototropic processes are theoretically feasible.